In recent years, secondary batteries such as lithium-ion secondary batteries, lithium-ion capacitors, and air cells have been actively developed. In particular, demand for lithium-ion secondary batteries with high output and high energy density has rapidly grown with the development of the semiconductor industry, for electronic devices, for example, portable information terminals such as cell phones, smartphones, and laptop computers, portable music players, and digital cameras; medical equipment; next-generation clean energy vehicles such as hybrid electric vehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles (PHEVs); and the like. The lithium-ion secondary batteries are essential as rechargeable energy supply sources for today's information society.
There is a very great need for more compact and higher capacity lithium-ion secondary batteries. Thus, electrodes formed of an alloy-based material of silicon, tin, or the like, instead of a carbon material such as graphite (black lead) which has been conventionally used as a negative electrode active material, have been actively developed. The (graphite has a theoretical capacity of 372 mAh/g, whereas the negative electrode of silicon has a dramatically high theoretical capacity of 4200 mAh/g, and therefore silicon is an optimal material for higher capacity lithium-ion secondary batteries.
However, when the material that is alloyed and dealloyed with lithium (e.g., silicon) greatly expands and contracts with reception and release of carrier ions in charge and discharge cycles; therefore, when the amount of carrier ions received by the material increases, the contact states between an active material and a conductive additive, between active materials, and between an active material and a current collector become worse and a conductive path is lost in some cases. The loss of the conductive path decreases the capacity as charge and discharge cycles increase. Moreover, in some cases, silicon is deformed or broken to be separated from a current collector or pulverized, so that a function as a lithium-ion secondary battery becomes difficult to maintain.
Patent Document 1 discloses a silicon layer that is formed over an uneven current collector so that a stress due to expansion or contraction of the silicon is reduced.